The present invention is broadly directed to a process for preparing a polyurethane foam. Blowing agents are used in the preparation of such foams for obtaining the required gross density. In addition, chlorofluoroalkanes used as blowing agents in the preparation of insulating foams improve the heat insulating properties of such foams. Charging at least one of the reactants with gas serves to form centers of nucleation for starting the foaming reaction. It has been found difficult to keep the readily volatile, liquid blowing agents in the component or reaction mixture to which it has been added. Considerable quantities of blowing agent are lost in the conventional method of injection mixing due to the sudden change from the injection pressure to the pressure in the mixing chamber. Further quantities of blowing agent are lost when the reaction mixture leaves the mixing chamber due to the reduction to atmospheric pressure. All blowing agent so lost is no longer available for the formation of foam. Thus in order to obtain the required density, it has generally been necessary to utilize higher quantities of blowing agent than would theoretically be necessary if no blowing agent were lost.
The problem arises of providing a process which enables the quantity of blowing agent used to be reduced while preserving the advantageous physical properties of the foam obtained as end product by ensuring that the blowing agent added is utilized as completely as possible for its intended purpose, namely adjustment of the product to the desired gross density and in some cases improvement in the heat insulating properties.